Electrical machines

ABSTRACT

Embodiments of the present invention provide an electrical machine comprising a first moveable element having a first plurality of permanent magnets associated therewith, a winding arranged generally adjacent to the first moveable element, wherein the winding is arranged to interact magnetically with a magnetic field of the permanent magnets associated with the first moveable element; a second plurality of permanent magnets; a second moveable element arranged adjacent to the first moveable element and having a plurality of a plurality of pole-pieces associated therewith, wherein the pole pieces are arranged to modulate the fields of the first and second pluralities of permanent magnets to enable magnetic coupling there-between, such that the first and second moveable elements move in a magnetically geared manner.

FIELD OF THE INVENTION

The present invention relates to electrical machines.

BACKGROUND TO THE INVENTION

Mechanical gearboxes are extensively used to match the operating speedof prime-movers to the requirements of their loads for both increasingthe rotational speed such as, for example, in a wind-powered generatorsor reducing rotational speed such as, for example, in an electric-shippropulsion arrangement. It is usually more cost and weight effective toemploy a high-speed electrical machine in conjunction with a mechanicalgearbox to achieve requisite speed and torque characteristics. However,while such a high-speed electrical machine in conjunction with amechanical gearbox allows high system torque densities to be realised,such mechanical gearboxes usually require lubrication and cooling.Furthermore, reliability can also be a significant issue. Consequently,direct drive electrical machines are employed in applications where amechanical gearbox cannot be used. Some direct drive electricalmachines, such as permanent magnet rotary/linear transverse-fluxmachines (TFM) have poor power factors which make them unsuitable forelectrical power generation and require higher converter volt-ampereratings for motor applications.

Recently, pseudo-direct drive electrical machines have been proposed bythe present inventors in which first and second moveable elements, suchas inner and outer rotors, interact in a magnetically geared manner viaasynchronous harmonics of first and second pluralities of permanentmagnets. Such an assembly is described in various embodiments in GB 2437 568 by the present inventors which is incorporated herein byreference.

It is an object of embodiments of the present invention to provideimprovements to such pseudo-direct drive electrical machines.

SUMMARY OF INVENTION

An aspect of embodiments of the present invention provides an electricmachine, comprising a first moveable element having a first plurality ofpermanent magnets associated therewith, a winding arranged generallyadjacent to the first moveable element, wherein the winding is arrangedto interact magnetically with a magnetic field of the permanent magnetsassociated with the first moveable element; a second plurality ofpermanent magnets; a second moveable element arranged adjacent to thefirst moveable element and having a plurality of a plurality ofpole-pieces associated therewith, wherein the pole pieces are arrangedto modulate the fields of the first and second pluralities of permanentmagnets to enable magnetic coupling there-between, such that the firstand second moveable elements move in a magnetically geared manner.

Preferably, the first moveable element is arranged to move at agreater-speed than the second moveable element. The first moveableelement may be driven by the magnetic influence of the winding.

The first moveable element is preferably a rotor. First and secondpluralities of permanent magnets are preferably disposed on interior andexterior surfaces of the rotor. The polarities of the permanent magnetsare preferably aligned. However, the two pluralities of permanentmagnets may not be aligned and may comprise different number ofpole-pairs. A plurality of magnets arranged on an exterior surface ofthe rotor may comprise a greater number of pole pairs than a pluralityof magnets interior to the rotor. In this case, the magnetic field ofthe permanent magnets interior to the rotor may cause movement of thesecond moveable element. The pluralities of permanent magnets carried bythe rotor may be separated by an annulus. Preferably the moveableelement comprises ferromagnetic material. Preferably, the ferromagneticmaterial is steel, e.g. laminated silicon iron, solid steel or siliconiron or a soft magnetic composite (pressed iron powder). Theferromagnetic material may form a structure of the first moveableelement.

The first moveable element may have a cup or bowl-like structure. Anannular rim-portion of the structure may support the first and secondpluralities of permanent magnets. The first moveable element may besupported perpendicularly to the rim. The first moveable element may besupported at one or both ends. Preferably the moveable element issupported upon one or more bearings. The annular portion of the moveableelement may be formed of a different material to a support portion. Thesupport portion may be non-magnetic. First and/or second pluralities ofpermanent magnets may be mounted upon a ferromagnetic member in the casethat the first moveable element is non-magnetic. The first moveableelement may or may not be connected to an output shaft. In the case thatthe first moveable element is not connected to an output shaft,construction of the machine is simplified. The second moveable elementis preferably connected to an output shaft.

A plurality of permanent magnets may be formed from one of an isotropicmaterial, an array of anisotropic magnet segments, or pre-alignedanisotropic material. The material may be NdFeB (neodymium, iron, andboron), SmCo (Samarium Cobalt) or Hard Ferrite (Strontium or BariumFerrites). The material may be epoxy bonded onto the first moveableelement or may form the moveable element.

The second moveable element may form an output or input (motor orgenerator) of the electric machine. The second moveable element ispreferably associated with a plurality of pole-pieces. Preferably, themachine further comprises a first stator associated with the secondplurality of permanent magnets. The pole pieces preferably couple themagnetic fields of the plurality of permanent magnets associated withthe first stator with those of the first moveable element. The machinemay further comprise a second stator associated with the winding.Preferably, the first stator is arranged concentrically within thesecond moveable element. Preferably, the second stator is arrangedaround the first moveable element. Preferably, the winding is arrangedabout the second stator adjacent to the first moveable element. That is,the winding and first moveable elements are preferably not interposed byanother member or element. The machine may preferably comprise 3 airgapsbetween moveable elements or moveable and static elements. Preferably,the first and second moveable elements are adjacent. That is, preferablynot interposed by a static element or stator.

An aspect of embodiments of the present invention provides an electricmachine, comprising a first moveable element having a ferromagneticmember mounted thereon and a plurality of permanent magnets supportedupon the ferromagnetic member, wherein the first moveable element isarranged to interact in a magnetically geared manner via the pluralityof permanent magnets, with a second moveable element; and a windingarranged to interact magnetically with a magnetic field of the pluralityof permanent magnets associated with the first moveable element.

An aspect of embodiments of the present invention provides an electricmachine, comprising: a first moveable element having a plurality ofpermanent magnets forming a Halbach array associated therewith, whereinthe first moveable element is arranged to interact in a magneticallygeared manner via the plurality of permanent magnets, with a secondmoveable element; a winding arranged to interact magnetically with amagnetic field of the plurality of permanent magnets associated with thefirst moveable element.

An aspect of embodiments of the present invention provides an electricmachine, comprising: a first moveable element having first and secondpluralities of permanent magnets associated therewith, wherein the firstmoveable element is arranged to interact in a magnetically gearedmanner, via the first plurality of permanent magnets, with a secondmoveable element, wherein the first and second pluralities havedifferent numbers of magnetic poles; and a winding arranged to interactmagnetically with a magnetic field of the second plurality of permanentmagnets associated with the first moveable element.

An aspect of the present invention provides an electric machine,comprising: a first moveable element having a plurality of permanentmagnets formed in a magnetised isotropic or anisotropic materialassociated therewith, wherein the first moveable element is arranged tointeract in a magnetically geared manner via the plurality of permanentmagnets, with a second moveable element; a winding arranged to interactmagnetically with a magnetic field of the plurality of permanent magnetsassociated with the first moveable element.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 shows an electric machine according to a first preferredembodiment of the present invention;

FIG. 2 shows an electric machine according to a second preferredembodiment of the present invention;

FIG. 3 shows a segment of a rotor according to a preferred embodiment ofthe present invention;

FIG. 4 shows a rotor according to the preferred embodiment of thepresent invention;

FIG. 5 shows a rotor according to another preferred embodiment of thepresent invention;

FIG. 6 shows rotors according to two further preferred embodiments ofthe present invention;

FIG. 7 shows two rotors according to still further preferred embodimentsof the present invention;

FIG. 8 shows preferred embodiments of rotor;

FIG. 9 shows another preferred embodiment of rotor;

FIG. 10 shows an axial field electric machine according to a preferredembodiment of the present invention;

FIG. 11 shows a preferred embodiment of a rotor used in the axial fieldelectric machine;

FIG. 12 shows a further preferred embodiment of an axial field electricmachine;

FIG. 13 shows a further preferred embodiment of axial field electricmachine;

FIG. 14 shows a preferred embodiment of a linear electric machine;

FIG. 15 shows a preferred embodiment of linear electric machine having atubular construction; and

FIG. 16 shows a further preferred embodiment of linear electric machinehaving a tubular construction.

Description of the preferred embodiments

FIG. 1 shows a first preferred embodiment of an electrical machineaccording to the present invention.

The first embodiment 100 comprises an inner stator 102 having aplurality of permanent magnets 104 mounted around, or carried upon, anouter periphery thereof. In the shown embodiment, 44 permanent magnetsforming 22 pole-pairs are carried upon the inner stator 102. However, itwill be realised that other numbers of permanent magnets and pole-pairsmay be utilised.

Arranged radially around the inner stator 102 is a first, or inner,rotor 106 which carries a plurality of pole-pieces 108. The pole pieces108 are mounted on or within a substrate 106. Alternatively, the polepieces 108 may substantially form the rotor 106, for example, bymounting on end plates or end-rings to form a cage-like supportingstructure. In the shown example, there are 26 pole pieces 108. However,it will be realised that embodiments of the invention are not limited tosuch number. A second, or outer, rotor 110 is arranged radially aroundthe outside of the inner rotor 106. As will be explained, the secondrotor 110 carries a plurality of permanent magnets 112 thereon. Asecond, or outer, stator 114 is disposed around the outer rotor 110 andforms an exterior wall of the machine 100. The second stator 114 has athree-phase winding 116 running there-through, although it will berealised that a winding 116 carrying another number of phases, such astwo or five phases, can be utilised.

FIG. 2 shows a second preferred embodiment of an electrical machineaccording to the present invention.

The second embodiment 200 comprises a first, inner, stator 202 throughwhich a multiple-phase, for example, three-phase, winding 204 isarranged. Disposed around the inner stator 202 is a first, inner, rotor206 which, as will be explained, carries a plurality of permanentmagnets 208. A second, outer, rotor 210, disposed around the inner rotor206, carries a plurality of pole-pieces 212. A second, outer, stator 214is disposed around a periphery of the apparatus and carries, on an innercircumference or periphery thereof, a plurality of permanent magnets 216forming a plurality of pole-pairs. In the preferred embodiment, thereare 22 pole-pairs formed by 44 permanent magnets 216. However, othernumbers of permanent magnets 216 may be utilised.

A brief description of the operation of the first and second embodiments100, 200 of electrical machines will now be provided.

The 3-phase windings 116, 204, and associated currents, are arranged tocreate magnetic fields that couple with the first or fundamentalharmonic of the magnetic field produced by the permanent magnets 112,208 associated with the rotor 110, 206 carrying a plurality of permanentmagnets 112, 208, in order to produce torque and the fundamentalharmonic of the permanent magnet 112, 208 array couples with the winding116, 202 to produce an electromotive force (EMF). That is, in the firstembodiment 100, the magnetic field produced by the winding 116 coupleswith a magnetic field of the permanent magnets 112 carried upon thesecond rotor 110. In the first embodiment illustrated, thefirst/fundamental harmonic corresponding to the permanent magnets 112has 4 pole-pairs. In the second embodiment, the magnetic field producedby the winding 204 couples with a magnetic field of the permanentmagnets 208 carried upon the first rotor 206.

The following describes the operation of the machine as a motor. Therotors 110 and 206 carrying the permanent magnets 112, 208 are caused torotate at a relatively high-speed by the current flow in the windings116, 204. In order to cause geared rotation of the other rotor in eachembodiment 106, 210 a coupling between the pluralities of fixed androtatable permanent magnets 112, 104 and 208, 216 respectively isrealised using the rotatable pole pieces 108, 212. The pole pieces 108,212 are used to allow the fields of the permanent magnets 112, 104 and208, 216 to interact. The pole pieces 108, 212 modulate the magneticfields of the permanent magnets 112, 104 and 208, 216 so they interactto the extent that rotation of one rotor 110, 206, caused by the currentflow in the windings, will induce rotation of the other rotor 106, 210in a geared manner. Rotation of the first rotor 110, 206 at a speed ω₁will induce rotation of the second rotor 106, 210 at a speed ω₂ whereω₁>ω₂ and visa versa. Consequently, the low-torque drive applied to thehigh-speed rotor 110, 206 is converted to a high-torque drive output bythe low-speed rotor 106, 210. This gearing allows the production of anelectrical machine capable of producing a high-torque to be madeconsequently smaller. In more detail, the pole pieces 106 modulate themagnetic field of the permanent magnets 112, 208. For the permanentmagnets 112, 208, for example, this results in a relatively largeasynchronous harmonic having the same number of poles as the permanentmagnets 104, 216, which enables coupling between the first 110, 206 andthe second 106, 210 rotors such that movement of one induces movement ofthe other, in a geared manner. Alternatively, when acting as agenerator, a low speed high torque mechanical drive (e.g. wind turbine)is connected to the pole piece rotor (108,212). The action of themagnetic gearing causes the rotatable permanent magnets (110,206) torotate at a higher speeds.

One skilled in the art understands how to select and design the polepieces 108, 212, given the first 112, 208 and second 104, 216pluralities of permanent magnets, to achieve the necessary magneticcircuit or coupling such that gearing between the first 110, 206 andsecond 106, 210 rotors results as can be appreciated from, for example,K. Atallah, D. Howe, “A novel high-performance magnetic gear”, IEEETransactions on Magnetics, Vol. 37, No. 4, pp. 2844-2846, 2001 and K.Atallah, S. D. Calverley, D. Howe, “Design, analysis and realisation ofa high performance magnetic gear”, IEE Proceedings—Electric PowerApplications, Vol. 151, pp. 135-143, 2004, which are incorporated hereinby reference for all purposes.

Advantageously, in embodiments of the present invention, efficiency ofthe electric machine is improved due to the location of a winding 116,202 being generally adjacent to a high-speed moveable element which itdrives 110, 206. Still further, the high-pole number permanent magnetarray 104, 216 is mounted upon a stator 102, 214 which provides a moreconvenient mounting arrangement for this plurality of permanent magnetsand avoids a need to contain the permanent magnets against centrifugalloads.

Referring to FIGS. 3 and 4, a portion of, and an entire, first preferredembodiment of a rotor 110, 206 utilised in the first and secondpreferred embodiments 110, 200 of electrical machine are respectivelyshown.

The rotor 110, 206 is formed by an annular ferromagnetic member 301. Theferromagnetic material is, in the preferred embodiment, steel, e.g.laminated silicon iron, solid steel or silicon iron or a soft magneticcomposite (pressed iron powder). Opposing faces of the member 301 havemounted thereon first and second pluralities of permanent magnets in anarray. A first plurality of permanent magnets 302 is mounted upon aninterior surface of the ferromagnetic member 301 and a second pluralityof permanent magnets 303 is mounted upon an exterior surface of theferromagnetic member 301. In the preferred embodiment, the first andsecond pluralities of permanent magnets 302, 303 have identical polenumbers and the extents of each pole in the respective magnet arrays arealigned. In the first embodiment of rotor 110, 206, there are 8permanent magnets forming the respective arrays of permanent magnets,producing a 4 pole-pair magnetic field. However, other numbers ofpermanent magnets and pole-pairs may be utilised.

The ferromagnetic member 110, 206 conducts magnetic flux from the innermagnet array 302 to the outer magnet array 303 and vice versa, withoutcreating any detrimental electromagnetic effects. Further, the design isnot sensitive to the thickness of the ferromagnetic member 110, 206 and,hence, the ferromagnetic member 110, 206 may be of sufficient thicknessto have a required physical strength for the annular rotor 110, 206. Theuse of steel, in the preferred embodiment, does not contribute to theoverall magnetic airgap and open surfaces of the magnets are conduciveto a high level of heat rejection to prevent the permanent magnets 302,303 from overheating. Advantageously, the rotor 110, 206 has goodmechanical strength, is cheap to produce and has a simple construction.

FIG. 5 shows a second preferred embodiment of a rotor 110, 206 for usein an electrical machine. Essentially, the second preferred embodimentof rotor has two permanent magnet arrays on opposing surfaces of aferromagnetic member which differ in pole-number.

The second embodiment of rotor 110, 206 comprises an annularferromagnetic member 501 having a first plurality of permanent magnets502 in the form of an array attached to an interior surface thereof. Asecond plurality of permanent magnets 503 is mounted in the form of anarray upon an opposing exterior surface thereof. In the second preferredembodiment, the numbers of permanent magnets forming the first andsecond pluralities 502, 503 are not equal. Consequently, the extent ofeach pole in the first and second pluralities 502, 500 does notcorrespond. That is, poles in each plurality do not have correspondingend-points. In the shown embodiment, the interior plurality 502comprises 8 permanent magnets forming 4 pole-pairs, whilst the exteriorarray comprises 16 permanent magnets forming 8 pole-pairs. There is a2:1 correspondence between the numbers of permanent magnets between theexterior 503 and interior pluralities of permanent magnets. It will berealised that other numbers of permanent magnets and pole-pairs may beutilised however.

The use of differing pole-number permanent magnet arrays allows adifferent number of poles to be used on the machine i.e. windingcoupling and gear i.e. pole-piece coupling elements of the machine. Thisallows the design of the machine and gearing magnetics to be decoupled.For example, a high pole-number machine may be required to achieve acertain frequency when the machine is acting as a generator, or tominimise back iron size, whilst a low gearing pole-number may berequired to achieve a predetermined gear ratio without requiring a veryhigh number of stationary magnets.

The use of a ferromagnetic member interposing first and secondpluralities of permanent magnets 302, 303, 502, 503 has furtheradvantages in terms of electrical machine design. In particular,simplified mounting of the rotor upon bearing supports is achieved.

FIG. 6 shows two further preferred embodiments of rotor construction andmounting structure. The two embodiments shown in FIG. 6 are mountedhaving single-ended support from bearings.

FIG. 6( a) shows a rotor 600 as in the first preferred embodiment shownin FIG. 3.

The rotor is shown in FIG. 6( a) in plan view and in FIG. 6( b) in sidecross-section through a central axis thereof. The rotor 600 comprises aferromagnetic member 601 having an annular portion upon which first 602and second 603 arrays of permanent magnets are mounted upon interior andexterior surfaces respectively. Extending inwardly from an end ofannular portion 601 a is a support portion 601 b. The support portion601 b is arranged perpendicular to the walls of the annular portion toprovide support thereto from a shaft 604 upon which the rotor 600 isrotatable mounted by bearings 605. The support portion 601 b extendsradially outward from the shaft 604 toward the annular portion 601 a andmay be in the form of arms, spokes, disc or an alternative supportingconstruction. The annular 601 a and support 601 b portions form a cup orbowl-shape when viewed in side-cross section. This constructionsimplifies manufacture of the rotor 600, assembly and disassemblythereof, for example, during construction or maintenance.

In the first embodiment, the rotor is constructed from a single unitarypiece of ferromagnetic material, such as steel, which reduces amanufacturing cost.

Referring to FIG. 6( c) there is shown a further preferred embodiment ofrotor 610. The rotor 610 has the same cup or bowl-shape as the firstembodiment, but is manufactured and constructed from two pieces ofdifferent material.

The rotor 610 is comprised of an annular ferromagnetic part 611 and asupport part 612. The support part extends from shaft-mounted bearings613 and supports the annular part 611 at a first, single, end thereof.The annular part carries first 614 and second 615 pluralities ofpermanent magnets on interior and exterior surfaces thereof. The annularand support parts 611, 612 are formed of different materials. Forexample, the ferromagnetic annular part 611 may be supported upon anon-magnetic support part 612, such as made from a composite material.This allows different materials to be used to improve magneticproperties of the rotor 610. This construction also allows theferromagnetic part 611 to be formed of a laminar construction and/or thesupport part to be solid, which would increase strength.

In the embodiments of rotor 600, 610 shown in FIGS. 6( a)-(c)single-ended support has been shown. However, it will be realised thatdouble-ended support from bearings arranged at either end of the rotormay be provided.

A construction of rotor 700 having a multi-layer or laminar constructionwill now be described with reference to FIG. 7. This embodiment of rotoris useful when permanent magnets are desired to be mounted upon backirons which are laminated, have poor mechanical strength, or when therotor is operated at high-speed.

As shown in FIG. 7, the rotor 700 comprises a support 701 having anannular part 701 a upon which pluralities of permanent magnets aremounted, as will be explained, and a support part 701 b which supportsthe annular part 701 a upon bearings 702 to be rotatable. As shown inFIGS. 7( b) and (c) the support part may be single or double-ended. Thatis, the annular part 701 a may be supported upon bearings at one or bothends thereof. The support 701 is manufactured from a material having ahigh-degree of mechanical strength and may be ferromagnetic, such assteel, or non-magnetic, such as a composite material.

Mounted upon interior and exterior surfaces of the annular part 701 aare ferromagnetic back irons 703. An array of permanent magnets 704, 705is mounted upon each of the back irons. This embodiment allows a laminarback iron to be utilised having poor mechanical strength or a support701 having a light weight, such as a composite material. Whilst shownhaving both permanent magnet arrays mounted upon back irons, anembodiment can be conceived in which the support part is ferromagneticand only a single back iron is provided for one of the permanent magnetarrays. Different numbers of poles may be provided in the interior andexterior magnet arrays.

FIG. 8 shows three preferred embodiments of rotors for use withelectrical machines. The rotors shown in FIG. 8 do not require a backiron and thus may be made from non-magnetic material havinghigh-strength e.g. for use in a large electrical machine, light-weightor a combination thereof. Examples of such materials are titanium, anInconel (™) alloy (high-strength) or a composite material (light-weight)such as carbon-fibre composite.

FIG. 8( a) shows a rotor 810 in plan-view comprising an annular part 811having first 812 and second 813 arrays of permanent magnets mounted uponinterior and exterior opposing surfaces thereof.

The arrays of permanent magnets 812, 813 are configured as a Halbacharray. A Halbach array is an arrangement of permanent magnets in which amagnetic field to one side of the array is enhanced whilst a magneticfield on an opposing side is cancelled. That is, the array isself-shielding, wherein a flux return path is within the permanentmagnet material itself. A back iron is not then required and the rotor810 may be manufactured from a non-magnetic material, or a materialhaving a lower ferromagnetic material content or thickness, particularlyin the case that the Halbach array is imperfect or an approximation to aHalbach array and not fully self-shielding.

The embodiment shown in FIG. 8( a) comprises discrete permanent magnetsarranged to form a Halbach array on each side of the annular part 811 ofthe rotor 810. The first and second arrays 812, 813 may have differentpole-numbers as in the shown embodiment.

Referring to FIG. 8( b) a rotor 820 is shown in which Halbach arrays ofpermanent magnets are formed in an isotropic material. The rotor 820comprises an annular part 821 having first 822 and second 823 magneticrings mounted upon interior and exterior opposing surfaces thereof. Suchrings may be made from epoxy bonded NdFeB. A single-shot magnetisingfixture, or similar, is utilised to imprint a Halbach self-shieldingmagnetisation pattern upon the magnetic rings 822, 823. The magneticrings 822, 823 may be manufactured as a unitary part, simplifyingconstruction, or as a plurality of parts or pieces. The magnetic rings822, 823 may be over-moulded on a support ring and then magnetised as asingle component. As will be realised, over-moulding is a process bywhich the magnetic rings would be moulded about a supporting ring orpart.

In a further embodiment 830, shown in FIG. 8( c), a Halbach array ofpermanent magnets is provided without a supporting structure. That is, arotor is formed of an isotropic material 831 without a support structurei.e. self-supporting. A single-shot impulse magnetising fixture, orsimilar, is utilised to imprint a pair of Halbach magnetisation patternsupon the magnetic ring to form first and second Halbach arrays. This isparticularly useful in machines desired to have light-weight rotorswhich are high in strength having low inertia. Such a ring may be madefrom a magnetically-loaded carbon fibre composite tube. The sametechnique may be used, as shown in FIG. 9, to produce a rotor 900 havingthrough-magnetisation, that is, conventionally directed pole-pairs. Thisavoids the need to construct a rotor 900 using discrete permanentmagnets. The ring may be made from a radially anisotropic, isotropicmaterial or a reinforced material such as magnetically-loaded carbonfibre composite tube.

The use of moveable members having arrays of permanent magnets carriedupon opposing faces in electrical machines is not limited to radialfield machines.

FIG. 10( a) shows an axial field electric machine 1000 comprising astator case 1001 having a static, inwardly-facing, high-pole numberarray of permanent magnets 1002 mounted upon an interior surface, anarmature/stator incorporating a multiphase winding 1003 mounted upon anopposing interior face of the stator case 1001, a low-speed,high-torque, rotor 1004 carrying a plurality of pole pieces 1009, ahigh-speed rotor 1005 carrying first 1006 and second 1007 arrays ofpermanent magnets upon opposed surfaces thereof, and an input/outputshaft 1008. It can be seen that the stator case features an inwardlydirected arm at either end thereof which are interposed by the rotors1004, 1005. FIG. 10( b) shows a cross-section through the machine at A-Ashown in FIG. 10( a). The arrangement of the high-pole number array ofpermanent magnets 1002 mounted upon the interior-facing surface of thestator case 1001 may be appreciated. FIG. 10( c) shows a cross sectionat line B-B through the rotor 1004 carrying the pole pieces 1009, whilstFIG. 10( d) shows a cross section along line C-C in FIG. 10( a) whichshows the second array 1007 of permanent magnets mounted upon thehigh-speed rotor 1005.

FIG. 11 shows the rotor 1005 of FIG. 10 in more detail. The rotor 1005is made from a disc-like ferromagnetic material, such as steel, having acentral aperture for mounting upon a shaft. The rotor 1005 is henceannular. A first plurality of permanent magnets 1006 is mounted on afirst face of the rotor 1005 and a second plurality of permanent magnets1007 is mounted upon a second face opposed to the first face. In theshown embodiment, the opposing faces carry 8 pole pairs with equalpole-pair of permanent magnets on each face. However, unequal numbers ofpole-pairs could be carried upon the faces.

FIG. 12 shows a second preferred embodiment of axial field electricmachine 1200 which reduces a normal force experienced on the pole-piecesor magnets by using a dual armature configuration.

The electric machine 1200 comprises a stator case 1201 having first andsecond static, inwardly-facing, high-pole number arrays of permanentmagnets 1202, 1203 mounted upon interior surfaces of the stator case1201 at either end of the machine. Proximal to the permanent magnetarrays 1202, 1203 at either end of the machine are a pair of low-speed,high-torque, rotors 1204, 1205 carrying a plurality of pole-pieces.Adjacent thereto, there is arranged a pair of high-speed rotors 1206,1207, each carrying first and second pluralities of permanent magnets onopposing faces, as shown in FIG. 11. Centrally within the machine areprovided stator armatures incorporating multiphase windings 1208, 1209and an input/output shaft 1210. It can be seen that the stator case 1201features inwardly extending arms 1201 a-c at either end and in a centrethereof. Each pair of arms 1201 a,b and 1201 b,c is interposed by inputand output rotors.

FIG. 13 shows a further embodiment of axial field electric machine. 1300in which stator armatures are arranged at opposing, outer, ends of themachine.

The electric machine 1300 comprises a stator case 1301 featuringinwardly extending arms 1201 a,c at either end and in a centre thereof1301 b. Mounted upon an interior, inwardly facing, surface of the arms1301 a, c are stator armatures incorporating multiphase windings 1208,1209. Adjacent thereto, there is arranged a pair of high-speed rotors1306, 1307, each carrying first and second pluralities of permanentmagnets on opposing faces. Interposing permanent magnet arrays 1302,1303 arranged upon opposing sides of arm 1301 b are a pair of low-speed,high-torque, rotors 1304, 1305 carrying a plurality of pole-pieces.

A linear electric machine featuring a pair of stators is shown in FIG.14. The machine 1400 comprises first and second stators 1401, 1402forming exterior walls at either side of the machine. Attached to aninterior facing surface of the first stator 1401 is a plurality ofpermanent magnets forming a first permanent magnet array 1403.Similarly, attached to an interior side of the opposing stator 1402 is awinding 1404. Interposing the stators 1401, 1402, magnet array 1403 andwindings 1404 is a low speed, high-torque, pole-piece armature 1405which is moveable in first and second opposed linear directions. Ahigh-speed armature 1406 carrying a plurality of permanent magnets issimilarly moveable in first and second opposed linear directions. It canbe seen that the winding 14304 is adjacent to the moveable elementcarrying a first plurality of permanent magnets, with which itmagnetically interacts to cause movement of that moveable element 1405.The plurality of permanent magnets carried by the high-speed moveableelement 1405 is coupled via the plurality of pole pieces to the secondplurality of permanent magnets 1403, such that movement of thehigh-speed armature 1406 causes geared movement of the low-speedarmature 1405. The magnet array 1406 may be formed as a pair of magnetarrays having different pole numbers mounted upon opposing faces of themoveable element.

Whilst the electric machine shown in FIG. 14 is planar, FIG. 15 shows atubular construction of a similar electric machine 1500 incross-sections aligned with and perpendicular to an axis of the machine.

The machine 1500 comprises a first tubular stator 1501 arranged at acentre of the machine and a second stator 1502 arranged to form an outerperiphery or case of the machine. Arranged around an exterior surface ofthe first stator 1501 is a plurality of permanent magnets forming afirst magnet array 1503 having a high-pole number. Adjacent to the firstmagnet array 1503 is pole-piece armature 1505 which encircles the firststator 1501 and magnet array 1503. The pole-piece armature 1505 is formsa low-speed armature and is moveable in first and second linearlyopposed directions, as shown. Arranged around the pole-piece armature1505 is a high-speed armature 1506 carrying one or two arrays ofpermanent magnets as in the embodiment described with reference to FIG.14. A winding 1504 is carried upon an interior surface of the statorcase 1502. Operation of the electric machine is as described withreference to FIG. 14.

A further embodiment of electric machine 1600 is shown in FIG. 16. Theelectric machine 1600 has a similar tubular arrangement to that shown inFIG. 15.

However, a tubular stator 1602 is arranged centrally to the machine 1600carrying a winding 1604 upon an exterior, outwardly facing, surfacethereof. Around the stator 1602 and winding 1604 is a linearly moveableelement 1606 having one or two arrays of permanent magnets carriedthereon. Around the moveable element 1606 is a further moveable element1605 having a plurality of pole-pieces associated therewith. An exterioror case of the electric machine 1600 is formed by a further stator 1601having a high pole-number array of permanent magnets 1603 arranged uponan interior surface thereof. Operation of the electric machine 1600 isas described with reference to FIGS. 14 and 15.

It will be appreciated that embodiments of the invention have beendescribed with reference to electrical machines. One skilled in the artappreciates that such electrical machines can be used as motors orgenerators. When so-used, applying a 3-phase supply to the windingsresults in a geared electrical motor. However, rotating one of therotors results in the electrical machine being used as a gearedgenerator. Furthermore, although the above embodiments have beendescribed with reference to using a 3-phase winding, embodiments are notlimited to such an arrangement. Embodiments can be realised in whichsome other form of winding such as, for example, a 2-phase windings, isused.

1. An electric machine, comprising: a first moveable element having afirst plurality of permanent magnets associated therewith, a windingarranged generally adjacent to the first moveable element, wherein thewinding is arranged to interact magnetically with a magnetic field ofthe permanent magnets associated with the first moveable element; asecond plurality of permanent magnets; a second moveable elementarranged adjacent to the first moveable element and having a pluralityof a plurality of pole-pieces associated therewith, wherein the polepieces are arranged to modulate the fields of the first and secondpluralities of permanent magnets to enable magnetic couplingthere-between, such that the first and second moveable elements move ina magnetically geared manner.
 2. The electric machine as claimed inclaim 1, comprising a first stator having the winding associatedtherewith.
 3. The electric machine as claimed in claim 1 or 2, whereinthe first stator is arranged adjacent to the first moveable element. 4.The electric machine as claimed in claim 1, 2 or 3, wherein the firststator is arranged around the first moveable element.
 5. The electricmachine as claimed in claim 1, 2 or 3, wherein the first stator isarranged interior to the first moveable element.
 6. The electric machineas claimed in any preceding claim, comprising a second stator having thesecond plurality of permanent magnets associated therewith.
 7. Theelectric machine as claimed in claim 6, wherein the second stator isarranged adjacent to the second moveable element.
 8. The electricmachine as claimed in claim 6 or 7, wherein the first and second statorsare interposed by the first and second moveable elements.
 9. Theelectric machine as claimed in any preceding claim, comprising threeair-gaps.
 10. The electric machine as claimed in any preceding claim,wherein the plurality of permanent magnets associated with the firstmoveable element has a first number of pole-pairs and the secondplurality of permanent magnets has a second number of pole-pairs whichis greater than the first number.
 11. The electric machine as claimed inany preceding claim, wherein the winding is arranged to interact with afundamental harmonic of the magnetic field of the plurality of permanentmagnets associated with the first moveable element.
 12. The electricmachine as claimed in any preceding claim, wherein the first pluralityof permanent magnets is arranged upon a first side of the first moveableelement and a third plurality of permanent magnets is arranged upon anopposing side of the first moveable element.
 13. The electric machine asclaimed in claim 12, wherein the first and third pluralities ofpermanent magnets have equal pole-numbers.
 14. The electric machine asclaimed in claim 12, wherein the first and third pluralities ofpermanent magnets have differing pole-numbers.
 15. The electric machineas claimed in claim 12, 13 or 14, wherein the first and second moveableelements are rotors and the first and third pluralities of permanentmagnets are disposed on interior and exterior surfaces of a first rotorrespectively.
 16. The electric machine as claimed in any precedingclaim, wherein the first and third pluralities of magnets are interposedby a ferromagnetic member.
 17. The electric machine as claimed in any ofclaims 12 to 16, wherein at least one of the first and third pluralitiesof permanent magnets are mounted upon a ferromagnetic back iron.
 18. Theelectric machine as claimed in claims 12 to 17, wherein the first andthird pluralities of magnets are formed in first and second rings of amagnetised isotropic material.
 19. The electric machine as claimed inclaim 18, wherein the rings are formed of NdFeB.
 20. The electricmachine as claimed in claims 12 to 17, wherein the first and thirdpluralities of magnets are formed in first and second rings of amagnetised anisotropic material.
 21. The electric machine as claimed inany of claims 12 to 20, wherein at least one of the first and thirdpluralities of permanent magnets forms a Halbach array.
 22. The electricmachine as claimed in any of claims 12 to 21, wherein the winding isarranged to interact with a fundamental harmonic of the magnetic fieldof one of the first and third pluralities of permanent magnets.
 23. Theelectrical machine as claimed in any preceding claim, wherein the firstand second moveable elements and first and second stators arecylindrically shaped, and concentrically disposed relative to an axis ofrotation thereby forming a radial field rotary electrical machine. 24.The electrical machine as claimed in any of claims 1 to 22, where thefirst and second moveable elements are at least one of annular or discshaped, and the first and second stators are axially disposed along theaxis of rotation thereby forming an axial field rotary electricalmachine.
 25. The electrical machine as claimed in any of claims 1 to 22,where the first and second moveable elements are elongate membersarranged to move in first and second linearly opposed directions,thereby forming a liner field electric machine.
 26. An electric machine,comprising: a first moveable element having a ferromagnetic membermounted thereon and a plurality of permanent magnets supported upon theferromagnetic member, wherein the first moveable element is arranged tointeract in a magnetically geared manner via the plurality of permanentmagnets, with a second moveable element; and a winding arranged tointeract magnetically with a magnetic field of the plurality ofpermanent magnets associated with the first moveable element.
 27. Theelectric machine as claimed in claim 26, wherein the first moveableelement is non-magnetic.
 28. The electric machine as claimed in claim 26or 27, wherein the moveable element comprises an annular portion havingthe ferromagnetic member mounted thereon.
 29. The electric machine asclaimed in claim 28, wherein the ferromagnetic member is mounted upon anexterior surface of the annular portion.
 30. The electric machine asclaimed in claim 28, wherein the ferromagnetic member is mounted upon aninterior surface of the annular portion.
 31. The electric machine asclaimed in any or claims 26 to 30, comprising a second ferromagneticmember having a plurality of permanent magnets supported thereon. 32.The electric machine as claimed in claim 31, wherein the first andsecond ferromagnetic members are supported upon opposing sides of themoveable element.
 33. The electric machine as claimed in any of claims26 to 32, wherein the first moveable element is rotatably supported at afirst end thereof.
 34. The electric machine as claimed in any of claims26 to 32, wherein the first moveable member is rotatably supported atfirst and second opposing ends thereof.
 35. An element for use in anelectric machine, comprising a ferromagnetic element having a pluralityof permanent magnets mounted thereon.
 36. The element as claimed inclaim 35, wherein the element is non-magnetic.
 37. The element asclaimed in claim 35 or 36, wherein the moveable element comprises anannular portion having the ferromagnetic element mounted thereon. 38.The element as claimed in claim 37, comprising a support portionarranged to support the annular portion upon a bearing.
 39. The elementas claimed in claim 38, wherein the support and annular portions areperpendicularly disposed.
 40. The element as claimed in any of claims 35to 39, comprising a second ferromagnetic member having a plurality ofpermanent magnets supported thereon.
 41. An electric machine,comprising: a first moveable element having a plurality of permanentmagnets forming a Halbach array associated therewith, wherein the firstmoveable element is arranged to interact in a magnetically geared mannervia the plurality of permanent magnets, with a second moveable element;a winding arranged to interact magnetically with a magnetic field of theplurality of permanent magnets associated with the first moveableelement.
 42. The electric machine as claimed in claim 41, the firstmoveable element having a second plurality of permanent magnets forminga Halbach array associated therewith.
 43. The electric machine asclaimed in claim 42, wherein the first and second Halbach arrays aredisposed upon opposing faces of the first moveable element.
 44. Theelectric machine as claimed in claim 42 or 43, wherein the first andsecond Halbach arrays comprise different pole-numbers.
 45. The electricmachine as claimed in claim 42, 43 or 44, wherein the first and secondHalbach arrays are arranged to form first and second opposed magneticfields.
 46. The electric machine as claimed in any of claims 41 to 45,wherein the Halbach array(s) are formed of discrete permanent magnets.47. The electric machine as claimed in any of claims 41 to 45, whereinthe Halbach array(s) are formed of a magnetised isotropic material. 48.The electric machine as claimed in any of claims 41 to 47, wherein thefirst moveable member is formed of a magnetised isotropic material andcomprises the permanent magnets forming the Halbach array(s).
 49. Anelement for use in an electric machine, comprising a plurality ofpermanent magnets forming a Halbach array.
 50. The element as claimed inclaim 49, comprising a second plurality of permanent magnets forming aHalbach array.
 51. The element as claimed in claim 50, wherein the firstand second pluralities of magnets are disposed on opposing surfaces ofthe member.
 52. The element as claimed in claim 49, 50 or 51, whereinthe element is one of a rotor, disc or an elongate member.
 53. Anelectric machine, comprising: a first moveable element having first andsecond pluralities of permanent magnets associated therewith, whereinthe first moveable element is arranged to interact in a magneticallygeared manner, via the first plurality of permanent magnets, with asecond moveable element, wherein the first and second pluralities havedifferent numbers of magnetic poles; and a winding arranged to interactmagnetically with a magnetic field of the second plurality of permanentmagnets associated with the first moveable element.
 54. The electricmachine as claimed in claim 53, wherein the first and second pluralitiesof magnets are disposed on opposing sides of the first moveable element.55. The electric machine as claimed in claim 53 or 54, wherein the firstand second moveable elements are rotors and the first and secondpluralities of permanent magnets are disposed on interior and exteriorsurfaces, respectively, of a first rotor.
 56. The electric machine asclaimed in claim 53, 54 or 55, wherein the first and second pluralitiesof magnets are interposed by a ferromagnetic member.
 57. The electricmachine as claimed in any of claims 53 to 56, wherein the first andsecond pluralities of magnets are formed in first and second rings ofmagnetised isotropic material.
 58. The electric machine as claimed inany of claims 53 to 57, wherein at least one of the first and secondpluralities of permanent magnets forms a Halbach array.
 59. The electricmachine as claimed in any of claims 53 to 58, wherein the secondmoveable element has a third plurality of permanent magnets associatedtherewith.
 60. The electric machine as claimed in any of claims 53 to59, comprising a plurality of pole-pieces arranged to modulate magneticfields produced by the first and third pluralities of permanent magnetsto cause magnetic coupling there-between.
 61. An element for use in anelectric machine, comprising first and second pluralities of permanentmagnets having differing numbers of poles.
 62. An electric machine,comprising: a first moveable element having a plurality of permanentmagnets formed in a magnetised isotropic or anisotropic materialassociated therewith, wherein the first moveable element is arranged tointeract in a magnetically geared manner via the plurality of permanentmagnets, with a second moveable element; a winding arranged to interactmagnetically with a magnetic field of the plurality of permanent magnetsassociated with the first moveable element.
 63. The electric machine asclaimed in claim 58, comprising a second plurality of permanent magnetsformed in the magnetised isotropic material.
 64. The electric machine asclaimed in claim 58 or 59, wherein the isotropic material is supportedupon the first moveable element.
 65. The electric machine as claimed inclaim 58 or 59, wherein the first moveable element comprises theisotropic material.
 66. A method of manufacturing an element for use inan electric machine, comprising magnetising one of an isotropic oranisotropic material.
 67. The method as claimed in claim 66, comprisingmagnetising the material to form a first array of permanent magnets. 68.The method as claimed in claim 67, comprising magnetising the materialto form a second array of permanent magnets.
 69. The method as claimedin any preceding claim 66, 67 or 68, wherein the array of permanentmagnets is a Halbach array.
 70. An element for use in an electricmachine, comprising a plurality of permanent magnets formed from amagnetised isotropic or anisotropic material.
 71. The element as claimedin claim 70, comprising a second plurality of permanent magnets formedin the magnetised isotropic or anisotropic material.